THE ORTHORHOMBIC STRUCTURE OF CaCO3, SrCO3, PbCO3 AND BaCO3: LINEAR STRUCTURAL TRENDS

Abstract

The crystal structures of four isostructural orthorhombic carbonates, CaCO 3 (aragonite), SrCO 3 (strontianite), PbCO 3 (cerussite), and BaCO 3 (witherite), were obtained by Rietveld refinements using data acquired by synchrotron high-resolution powder X-ray diffraction (HRPXRD). For BaCO 3 , powder neutron-diffraction data were obtained and refined by the Rietveld method. For aragonite, we also carried out a refinement of the structure by single-crystal X-ray diffraction. These carbonates belong to the space group Pmcn, with Z = 4. The CO 3 group is slightly non-planar, and the two independent C-O distances are slightly different. The CO 3 group becomes more symmetrical and less aplanar from CaCO 3 to BaCO 3 (M 2+ radii : Ca < Sr < Pb < Ba). The CaCO 3 structure is, therefore, the most distorted, whereas the BaCO 3 structure is the least distorted. Several linear structural trends are observed in plots of selected parameters as a function of the unit-cell volume, V. These parameters are radii of the nine-coordinated M 2+ cations, the unit-cell axes, the average <M-O> and <C-O> distances, average <O-C-O> angle, and aplanarity. These linear trends are the result of the effective size of the divalent ionic radius of the M cations that are coordinated to nine oxygen atoms. The geometrical features of the CO 3 group can be obtained reliably only by using neutron-diffraction data, especially in the presence of other heavy atoms.Keywords: CaCO 3 , aragonite, SrCO 3 , strontianite, PbCO 3 , cerussite, BaCO 3 , witherite, Rietveld refinements, synchrotron highresolution powder X-ray diffraction (HRPXRD), neutron diffraction, crystal structure. SOMMAIreNous avons affiné la structure cristalline de quatre carbonates orthorhombiques isostructuraux, CaCO 3 (aragonite), SrCO 3 (strontianite), PbCO 3 (cérusite), et BaCO 3 (witherite), par la méthode de Rietveld en utilisant les données acquises par diffraction X à haute résolution avec rayonnement synchrotron. Pour le BaCO 3 , nous avons utilisé des données obtenues par diffraction neutronique, aussi affinées par la méthode de Rietveld. Pour l'aragonite, nous avons affiné la structure par diffraction X sur monocristal. Ces carbonates répondent au groupe spatial Pmcn, avec Z = 4. Le groupe CO 3 est légèrement non planaire, et les deux distances C-O indépendantes diffèrent légèrement. Le groupe CO 3 devient plus symétrique et moins aplanaire en allant de CaCO 3 à BaCO 3 (M 2+ rayon : Ca < Sr < Pb < Ba). La structure de CaCO 3 serait donc la plus difforme, tandis que la structure de BaCO 3 serait la moins difforme. Plusieurs tendances structurales sont évidentes dans les tracés de paramètres choisis en fonction du volume de la maille élémentaire, V. Parmi ces paramètres sont le rayon des cations M 2+ à coordinence neuf, les dimensions le long des axes de la maille élémentaire, les distances <M-O> et <C-O> moyennes, l'angle <O-C-O> moyen, et l'aplanarité. Ces tracés linéaires sont le résultat de la dimension effective du rayon ionique du cation M bivalent coordonné aux ne...

Postaragonite phases of CaCO3 at lower mantle pressures

“…Structural ramifications of the ionic size ratio in M CO 3 aragonite group crystals (M = Ca, Sr, Ba, Pb) is exemplified by observations made by Ref. 29, where combined high-resolution X-ray diffraction and neutron diffraction record an increasing degree of disorder in CO 2− 3 units with decreasing cation size, with CaCO 3 having the largest variation in C-O bond lengths as well as a deviation from truly planar carbonate ions as a result of steric effects. Indeed, in the case of MgCO 3 , this ionic size effect inhibits the formation of an aragonite phase anywhere in its phase diagram and stabilises the R3c structure up to 85 GPa.…”

Structural, electronic, optical and bonding properties of strontianite, SrCO3: First-principles calculations

“…The crystallographic data of strontianite with orthorhombic symmetry and space group Pmcn [19] were taken as the initial lattice parameters (a ¼5.107 Å, b¼ 8.414 Å, c ¼ 6.027 Å) in the optimized process. A conventional cell of four formula units (Z¼4) is considered, instead of the primitive cell.…”

“…Since the crystal structure of strontianite was firstly reported by Bragg [1], a considerable amount of works have been developed to investigate the properties of strontianite, such as the crystal structure [17][18][19], crystal growth [20], solubility [21], thermodynamic properties [22], interfacial properties [23], FT-Raman and infrared spectroscopy [24][25][26], and luminescence [14,15,27]. The structural, electronic, optical, and bonding properties of strontianite are helpful to further understand the physical properties of strontianite.…”

The Effect of Chloride Ions on the Activity of Cerussite Surfaces

“…All computations were based on the cerussite crystal structure reported by Antao and Hassan [14]. The cerussite (110) surface was used for surface structure and property calculation.…”